Generally, a high-power amplifier has a characteristic of increasing its output power with an increase in its input power, and of saturating at a certain level of the output power.
The efficiency of the high-power amplifier has a characteristic of increasing near the saturation and reducing at low output power.
Therefore the high-power amplifier has a problem of reducing its efficiency at the low output power.
A mobile phone that adopts a communication scheme such as W-CDMA and N-CDMA controls its output power in accordance with the distance between the mobile phone and a base station and with radio wave conditions. The mobile phone, however, is more likely to carry out its transmission at the output power 10-15 dB less than the maximum output power.
Thus, it is necessary for the high-power amplifier used by the mobile phone to increase its efficiency (to reduce power consumption) not only at the maximum output, but also at the output power 10-15 dB less than the maximum output to increase talk time.
A conventional high-power amplifier makes a contrivance to increase the efficiency at the low output power by a design for decreasing an idle current.
However, since it is insufficient, a high-power amplifier is proposed which tries to increase the efficiency at the low output power by controlling the drain voltage or collector voltage of an amplifying element in response to the output power (see Non-patent Document 1, for example).
The following is the details.
A signal input from an input terminal is supplied to an amplifying element via an input matching circuit composed of a DC-cut capacitor, capacitor and inductor, and is amplified. The signal output from the amplifying element is output from an output terminal via an output matching circuit composed of a DC-cut capacitor, capacitor and inductor.
At the low output power, the efficiency of the high-power amplifier is increased by reducing the voltage output from a DC-DC converter by varying the value of a variable resistor.
Non-patent Document 1: T. B. Nishimura, N. Iwata, and G. Hau, “IEEE MTT-S Symp. Digest”, 1999, pp. 1091-1094.
With the foregoing configuration, the conventional high-power amplifier can improve the efficiency at the low output power to some extent by reducing the voltage output from the DC-DC converter by varying the value of the variable resistor. However, since the DC-DC converter is bulky, it offers a problem of being difficult to miniaturize the high-power amplifier, and of increasing its cost. In addition, the efficiency of the DC-DC converter itself reduces the overall efficiency, offering a problem of impairing the efficiency at the low output power.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a high-power amplifier capable of increasing the efficiency at the low output power without increasing the cost or size.